There are known in the prior art various arrangements for determining the validity of currency notes. Further as is known in the art, the U.S. currency notes are printed at least in part with ink containing magnetic particles. Many of the validators of the prior art employ magnetic techniques for validating notes.
One example of a currency validator of the prior art which relies on the magnetic character of the ink with which the notes are printed is my prior U.S. Pat. No. 5,068,519 issued Nov. 26, 1991. The apparatus disclosed in my prior patent produces a first signal as a measure of the saturation magnetization of a portion of the document printed with magnetic ink and a second signal as a measure of the remanent magnetization of the portion. The ratio of the second signal to the first provides a measure of the genuineness of the document.
While the apparatus shown in my prior patent is entirely satisfactory in determining the magnetic characteristics of a valid currency note, some counterfeits such as represented by Interpol's indicatives 12A14342E, 12A7513, 12A14342AV and others are magnetically indistinguishable from genuine currency notes. While such counterfeits may incorporate printing defects, the visual characteristics of the notes are extremely difficult to discern in the context of automated high speed currency counting operations.
Counterfeits of the type mentioned hereinabove are printed in part with ink which weakly conducts electrical current whereas genuine currency is printed with inks that are essentially non-conductive.